1. Field of the Invention
The present invention relates to a mixer circuit and, more specifically, to a mixer circuit for mixing first and second signals having mutually different frequencies to output a third signal.
2. Description of the Background Art
Conventionally, a mixer circuit for mixing two signals v.sub.1 and v.sub.2 having mutually different frequencies has been used in the field of communication. The mixer circuit forms a part of a multiplier circuit, a modulator, a phase detector and the like.
FIG. 7 is a schematic block diagram showing a structure of a conventional Gilbert type mixer circuit. Referring to FIG. 7, the mixer circuit includes a constant current source 50, NPN bipolar transistors 51 to 56, first input terminal pair 61a, 61b, second input terminal pair 62a, 62b and output terminal pair 63a, 63b.
The first pair of input terminals 61a and 61b receive mutually complementary signals V.sub.1+ and V.sub.1- (where v.sub.1 =v.sub.1+ -v.sub.1-). The second pair of input terminals 62a and 62b receive mutually complementary signals v.sub.2+ and v.sub.2- (where v.sub.2 =v.sub.2+ -v.sub.2-). Transistors 51 and 52, 53 and 54, and 55 and 56 form differential transistor pairs, respectively.
More specifically, transistors 51 and 52 have their bases connected to input terminals 61a and 61b, and emitters connected commonly to each other and to a line of a ground potential GND through constant current source 50. Constant current source 50 causes flow of a constant current 2I.sub.EE.
Transistors 53 and 54 have their bases connected to input terminals 61a and 61b, respectively, and emitters commonly connected to each other and to the collector of transistor 51.
Transistors 55 and 56 have their gates connected to input terminals 61b and 61a, respectively, and emitters connected commonly to each other and to the collector of transistor 52.
Transistors 53 and 55 have their connectors connected commonly to each other and to output terminal 63a, and transistors 54 and 56 have their collectors connected commonly to each other and to output terminal 63b.
The operation of mixer circuit shown in FIG. 7 will be described. Signals v.sub.1+ and v.sub.1- are converted to collector currents of transistors 51 and 52 and amplified, by the differential transistor pair consisting of transistors 51 and 52. The collector current of transistor 51 serves as the emitter current (base current) of the differential transistor pair consisting of transistors 53 and 54. The collector current of transistor 52 serves as the emitter current of differential transistor pair consisting of transistors 55 and 56.
Signals v.sub.2+ and v.sub.2- are converted to collector currents of transistors 53 and 54 and amplified by differential transistor pair consisting of transistors 53 and 54, and converted to collector currents of transistors 56 and 55 and amplified by the differential transistor pair consisting of transistors 55 and 56. The sum of collector currents of transistors 53 and 56 is provided as an output current i.sub.out+, and sum of collector currents of transistors 54 and 55 is provided as an output current i.sub.out-. Therefore, output currents i.sub.out+ and i.sub.out- have mutually complementary waveforms mixing signals v.sub.1 and v.sub.2.
This relation is represented by the following equations. When thermal voltage is represented by V.sub.T, collector currents i.sub.c51 to i.sub.c56 of transistors 51 to 56 are represented by the following equations (1) to (6), respectively. ##EQU1##
From equations (1) to (6), the relations between collector currents i.sub.c53 to i.sub.c56 and input signals v.sub.1 and v.sub.2 can be represented by the following equations (7) to (10). ##EQU2##
Here, it holds that i.sub.out =i.sub.c53 +i.sub.c55, i.sub.out=ic54 +i.sub.c56. The difference i.sub.out+ -i.sub.out- between the output currents i.sub.out+ and i.sub.out- is represented by the following equation (11). ##EQU3##
Here, series expansion of tanhx results in tanhx=x-x.sup.3 /3, and therefore it holds that tanhx.apprxeq.x where x is sufficiently smaller than 1. Therefore, the relation between input signals v.sub.1 and v.sub.2 and output currents i.sub.out+ and i.sub.out- can be represented as i.sub.out+ -i.sub.out- .apprxeq.2I.sub.EE (v.sub.1 /2V.sub.T)(v.sub.2 /2v.sub.T).
In other words, by calculating the difference between output currents i.sub.out+ and i.sub.out- and multiplying the difference by a constant, input signals v.sub.1 and v.sub.2 can be multiplied.
When input signals v.sub.1 and v.sub.2 are sinusoidal waves of mutually different frequencies f.sub.1 and f.sub.2, then the product v.sub.1 v.sub.2 of the two signals is given by Asin(f.sub.1 +f.sub.2)t+Bsin(f.sub.1 -f.sub.2)t (where A and B are constants). Therefore, by separating the component Asin(f.sub.1 +f.sub.2)t from the component Bsin(f.sub.1 -f.sub.2)t by means of a filter, it is possible to obtain a signal having the frequency (f.sub.1 +f.sub.2) which is the sum of the frequencies of two signals v.sub.1 and v.sub.2, or a signal having the frequency (f.sub.1 -f.sub.2) which is the difference between the frequencies of the two signals v.sub.1 and v.sub.2.
However, in the conventional mixer circuit, it has been difficult to reduce the power supply voltage, as it employs vertically connected two stages of differential transistor pairs.